The disposal of many waste water streams presents environmental problems because of organic pollutants contained in the waste water. One method in which these waste water streams are treated in order to facilitate their disposal involves a biological degradation or conversion of the organic pollutants to nonoffending products and/or easily removable products. This is accomplished by contacting the stream with a biomass of microorganisms capable of biologically degrading or converting the pollutants to carbon dioxide and other products.
In an anaerobic, methanogenic process the microorganisms are anaerobic bacteria which convert the organic pollutants to mainly carbon dioxide and methane. The methane is valuable as an energy source. In some processes wherein not only degradation of organic pollutants occurs but denitrification is also accomplished, the organic pollutants are converted to carbon dioxide (no methane being produced) while combined nitrogen is converted to easily removable molecular nitrogen.
Various devices have been disclosed for conducting the biological degradation processes. One type of device is the lagoon which is merely a large pond or lake opened to the air. The waste water is placed in the lagoon and retained for several months, during which time both aerobic and anaerobic bacteria decompose the pollutants. The top layer of the lagoon will be aerobic in nature while the bottom will be anaerobic in nature. These lagoons are not suitable in most situations because of the large area needed, the odor produced, and their slow conversion time. Any methane produced is also, of course, lost entirely.
Another type of device in which biological degradation may be conducted in the absence of added molecular oxygen is a digester. A conventional digester is simply a closed vessel, with provisions for mild agitation and sized to provide at least 10 days detention time. The anaerobic digesters are practical only for high strength wastes such as thickened domestic sludge and certain food processing waste. They can be operated reasonably well when the feed contains solids, but they are relatively ineffective for effluents which contain only soluble pollutants.
The most popular device for treating waste water streams containing soluble organic pollutants is the device known as the anaerobic filter. It consists of a packed tower containing packing on which is disposed a biomass of suitable microorganisms. In those anaerobic filters heretofore disclosed, the effluent or waste water stream was passed upward under plug flow conditions through the packed section, fresh feed being passed to the bottom of the packed tower with liquid overflow being removed at a point above the packed section. Under such conditions of operation, the packed tower would be filled with liquid at all times with the liquid level in the tower being maintained above the upper end of the packed section. Because, under such conditions of operation, the tower will be filled with liquid at all times, the tower must be constructed of a heavy and strong material (such as steel) which is relatively expensive.
A device similar to the anaerobic filter is also known in prior art for conducting an aerobic biological degradation process. More specifically it has been disclosed that an aerobic biological degradation process may be conducted by distributing the process liquid over the top of an open, packed vessel containing packing on which is located aerobic microorganisms, and allowing the liquid to trickle downwardly through the packing and in contact with the aerobic microorganisms. Being open to the air, these packed vessels would not be suitable for processes which must be conducted in the absence of molecular oxygen. The aerobic processes are not as desirable as the anaerobic processes because no methane, a potential energy source, is produced, the main products being carbon dioxide and water.
The anaerobic filter device described above is very useful in the treatment of waste water streams which contain no appreciable amount of solids, that is those in which the pollutants are soluble. Even though extensive research and development work has been done on the anaerobic filter devices, various problems still exist with their use. The cost of constructing a filter vessel using the upward plug flow as practiced in the prior art is mentioned above. Another problem with the anaerobic filter devices, as utilized in the prior art, is that there is a tendency, particularly when the linear velocity of liquid through the filter is quite low, for the biomass in the lower, or inlet portion of the filter to become so lush in its growth habit as to clog the filter. Further, if the linear velocity of the waste water stream through the filter is too high, such can result in inhibition or destruction of the biomass because the concentration of the pollutants will be increased above the level which can be tolerated by the microorganisms.
It is an object of the present invention to provide a new and useful biological degradation process for the treatment of waste water streams, and a new and useful apparatus therefor. It is an additional object of the present invention to provide a new and useful methanogenic biological degradation process of the anaerobic filter-type, and apparatus therefor, which utilizes relatively lightweight and inexpensive materials of construction. It is a still further object of the present invention to provide a new and useful anaerobic filter process and apparatus wherein the distribution of the process liquid through the filter is relatively simple, and which will readily free itself from accumulated biomass or other solids. Another object of the present invention is to provide an anaerobic filter process and apparatus which, unlike the prior-art anaerobic filter processes and apparatus, will handle and degrade some types of solid pollutants. Additional objects will become apparent from the following description of the invention.